1. Field of the Invention
The present invention relates to a magneto-optical memory element, wherein information is recorded into, read out from, or erased from the magneto-optical memory element by applying a laser beam onto the magneto-optical memory element.
2. Description of the Prior Art
Recently, an optical memory system has been developed, which ensures a high density storage, a large capacity storage, and high speed accessing. Especially, a magneto-optical memory element is on the way of the practical application, which includes a magnetic recording film having an axis of easy magnetization perpendicular to its surface. Such a magneto-optical memory element ensures erasing of old information and recording thereto new information. However, the magneto-optical memory element generally shows a low level of a reproduced signal. Especially, when the reproduction system is constructed to utilize the Kerr rotation angle included in the beam reflected from the magneto-optical memory element, the small Kerr rotation angle prevents the enhancement of the S/N ratio.
A polycrystal material such as MnBi or MnCuBi, an amorphous material consisting of rare earth element and transition metal such as GdCo, GdTbFe or TbFe, and a single crystal compound material such as GIG are known as materials suited for the magnetic recording film having an axis of easy magnetization perpendicular to its surface. However, these materials do not show a satisfying S/N ratio in the light beam reflected therefrom. In these materials, the amorphous material consisting of rare earth element and transition metal may be most preferable because the amorphous material ensures a stable fabrication of a large size magnetic recording film. Furthermore, the magnetic recording film made of the amorphous material consisting of rare earth element and transition metal can provide a large coercive force, and ensures a stable writing operation of fine bits. Moreover, in the magnetic recording film made of the amorphous material consisting of rare earth element and transition metal, the Curie temperature can be selected at a desired level.
An idea has been proposed to add cobalt to an alloy consisting of rare earth element and iron in order to increase the magneto-optical effect. For example, when cobalt (Co) is added to the alloy TbFe of rare earth element and iron, the Kerr rotation angle increases from 0.13.degree. (TbFe alone) to 0.28.degree. (TbFe+Co). However, by adding cobalt (Co) to the alloy (TbFe), the Curie temperature increases extremely from 120.degree. C. (TbFe alone) to over 300.degree. C. (TbFe+Co). When the Curie temperature rises, the recording sensitivity is reduced because the writing temperature becomes high.